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The findIndex()
method returns an index in the array, if an element in the array satisfies the provided testing function. Otherwise -1 is returned.
See also the find()
method, which returns the value of a found element in the array instead of its index.
Syntax
arr.findIndex(callback[, thisArg])
Parameters
callback
- Function to execute on each value in the array, taking three arguments:
element
- The current element being processed in the array.
index
- The index of the current element being processed in the array.
array
- The array
findIndex
was called upon.
thisArg
- Optional. Object to use as
this
when executingcallback
.
Return value
An index in the array if an element passes the test; otherwise, -1.
Description
The findIndex
method executes the callback
function once for every array index 0..length-1
(inclusive) in the array until it finds one where callback
returns a truthy value (a value that coerces to true
). If such an element is found, findIndex
immediately returns the index for that iteration. If the callback never returns a truthy value or the array's length
is 0, findIndex
returns -1. Unlike some other array methods such as Array#some, in sparse arrays the callback
is called even for indexes of entries not present in the array.
callback
is invoked with three arguments: the value of the element, the index of the element, and the Array object being traversed.
If a thisArg
parameter is provided to findIndex
, it will be used as the this
for each invocation of the callback
. If it is not provided, then undefined
is used.
findIndex
does not mutate the array on which it is called.
The range of elements processed by findIndex
is set before the first invocation of callback
. Elements that are appended to the array after the call to findIndex
begins will not be visited by callback
. If an existing, unvisited element of the array is changed by callback
, its value passed to the visiting callback
will be the value at the time that findIndex
visits that element's index; elements that are deleted are not visited.
Examples
Find the index of a prime number in an array
The following example finds the index of an element in the array that is a prime number (or returns -1 if there is no prime number).
function isPrime(element, index, array) { var start = 2; while (start <= Math.sqrt(element)) { if (element % start++ < 1) { return false; } } return element > 1; } console.log([4, 6, 8, 12].findIndex(isPrime)); // -1, not found console.log([4, 6, 7, 12].findIndex(isPrime)); // 2
Polyfill
This method has been added to the ECMAScript 6 specification and may not be available in all JavaScript implementations yet. However, you can polyfill Array.prototype.findIndex
with the following snippet:
if (!Array.prototype.findIndex) { Array.prototype.findIndex = function(predicate) { 'use strict'; if (this == null) { throw new TypeError('Array.prototype.findIndex called on null or undefined'); } if (typeof predicate !== 'function') { throw new TypeError('predicate must be a function'); } var list = Object(this); var length = list.length >>> 0; var thisArg = arguments[1]; var value; for (var i = 0; i < length; i++) { value = list[i]; if (predicate.call(thisArg, value, i, list)) { return i; } } return -1; }; }
Specifications
Specification | Status | Comment |
---|---|---|
ECMAScript 2015 (6th Edition, ECMA-262) The definition of 'Array.prototype.findIndex' in that specification. |
Standard | Initial definition. |
ECMAScript 2017 Draft (ECMA-262) The definition of 'Array.prototype.findIndex' in that specification. |
Draft |
Browser compatibility
Feature | Chrome | Firefox (Gecko) | Internet Explorer | Edge | Opera | Safari |
---|---|---|---|---|---|---|
Basic support | 45.0 | 25.0 (25.0) | No support | Yes | No support | 7.1 |
Feature | Android | Chrome for Android | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|
Basic support | No support | No support | 25.0 (25.0) | No support | No support | 8.0 |